HIV infection is associated with hematopoietic abnormalities, including direct infection of noncommitted (CD34+) or committed cell populations, alterations in cell function and/or number, and subsequent perturbations in cytokine production. We are currently characterizing effects of systemic cytokine administration on hematopoiesis in non-infected and SIV-infected fetuses and dams, and assessing the long-term effects of cytokine exposure on hematopoiesis and outcome with SIV infection in utero. Initial studies have focused on characterizing endogenous levels of stem cell factor (SCF) in the circulation of rhesus fetuses and dams. SCF is a hematopoietic cytokine active during early stages of hematopoiesis, and is known to induce proliferation of pluripotent progenitors (CD34+). Blood samples were collected from gravid adults every 10 days from early gestation to term (N=40). Fetal blood samples were collected by ultrasound-guided cardiocentesis every 10 days during the second and third trimesters, then from infants at birth and weekly up to two months postnatally (N=25). Assessments of adult sera indicated a decline in SCF concentrations during early pregnancy (0.9110.04 ng/ml [nongravid], 0.5110.02 ng/ml[gravid]). These levels remained relatively constant during gestation, with modest elevation close to term. Fetal circulating levels were much higher (2.3610.01 ng/ml) compared to maternal levels during all time points studied. A modest elevation was noted in the newborn, followed by an approximate two-fold decline by two months postnatal age (1.3210.07 ng/ml). These studies show that fetal monkey SCF levels are considerably higher when compared to adults', and circulating SCF concentrations in both fetus and dam do not change appreciably during the second and third trimesters. Studies focusing on effects of chronic maternal administration of SCF during gestation are currently in progress. *KEY*Fetal hematopoiesis, Stem cell factor, Pregnancy, Neonate